The present invention relates to ion sources, and more particularly, to a device, apparatus, system and method utilizing a field ionization phenomenon for the generation of positively charged ion beam.
The production of particle beams is of considerable importance in such diverse areas as atomic, molecular and plasma physics, thin film deposition, surface etching, ion implantation, submicron lithography, nano-electro-mechanical and nanophotonic system construction, new material synthesis, and electric propulsion devices. Applications utilizing anionic particulate beams find use in fundamental science areas, e.g., surface chemistry and catalysis, organic chemistry, and biology. For example, FAB (Fast Atom Bombardment) and TOF-SIMS (Time Of Flight Secondary Ion Mass Spectrometry) instruments are widely used for tailoring and analyzing new biomaterials and organic structures on the molecular level in the fields of pharmacology and biotechnology, and FIB (Focused Ion Beam) instruments are used for nano-processing and nano-patterning of solid surfaces.
It is recognized that modern processes employing particle beams require high current density, high brightness and high probe current. Such properties can be achieved, e.g., using ion guns employing a liquid metal ion source (LMIS). In one approach, a structure having a sharp tip is covered with a layer of liquid metal. Application of a negative high voltage to an extraction electrode in proximity to the tip brings about concentration of an electric field at the tip. When the voltage reaches a certain threshold value, the liquid metal located at the tip forms a conical cusp called Taylor Cone, leading to an extraction of ions from the tip. As the ions are emitted from the source, more liquid metal flows from a reservoir down the needle to the cusp to replenish the emitted material. Ions emitted from a LMIS-based ion gun can be focused in an extremely fine and very high density beam of a sub-micron diameter.
Another type of ion source is disclosed in U.S. Pat. Nos. 6,265,722 and 6,429,439. This type of source includes a needle, an extraction electrode disposed proximate to the tip of the needle and a voltage which maintains the tip at a high potential relative to the extraction electrode. A heated reservoir containing an organic ion source material contacts the needle such that the temperature of the organic ion source material is maintained at a magnitude sufficient to encourage capillary flow of the material from the reservoir along the needle. Such ion source, however, suffers from a severe limitation because, in practice, the applied heat to the reservoir is not sufficient to replenish the emitted material at the tip of the needle where ionization takes place.
Recently, fullerene-based ion guns have been developed. For example, International Patent Application, Publication No. WO2006/056975, the contents of which are hereby incorporated by reference, discloses apparatus in which fullerene molecules passing through a heated duct are negatively charged by a process of low-energy electron capture. The charged molecules are manipulated to an anionic beam by one or more electrodes.
Fullerenes, most notably C60, are a newly discovered form of carbon. The fullerenes are a family of hollow (cage) all-carbon structures. C60 is the most prominent member of this family. C60 is a perfectly symmetrical molecule composed of 60 carbon atoms arranged on the surface of a sphere in an array of 12 pentagons and 20 hexagons (a soccer-ball molecule). C60 has many unique properties but most relevant here are its structural rigidity (closed cage) and its thermal and collisional stability. Other relatively common fullerenes are C70, C76 and C84. Their structure is described in “Science of fullerenes and carbon Nanotubes,” M. S. Dresselhaus et al., Academic Press, San-Diego 1996, the contents of which are hereby incorporated by reference. Fullerene cages are approximately 7-15 Angstroms in diameter. The molecules are relatively stable; the molecules dissociate at temperatures above 1000° C. Fullerenes sublime at much lower temperatures, typically a few hundred degrees centigrade.
The use of energetic cluster or polyatomic ions as primary projectiles for static SIMS analysis of organic and inorganic samples and for depth profiling (dynamic SIMS) of such samples has many advantages compared to the traditionally used atomic ion collier. Polyatomic or cluster ions produce significantly higher yield of secondary ions (5-100 times) as compared to atomic ions. This yield enhancement relates to the fact that the deposited impact energy is distributed over a broader and shallower surface region than for an atomic species, and only the topmost layers of the substrate absorbs the impact energy. Therefore, the use of fullerene ion projectiles as the primary beam is attractive due to the relatively shallow penetration of the fullerene ion projectile into the bulk and the extremely high surface sensitivity of the adsorbed molecules analysis. Also, depth profiling of soft matter (e.g., organic, polymeric or biomaterial) is more probable due to reduced sub-surface damage and reduced interlayer mixing.
Various methods for the generation of fullerene ion beams are known. Representative examples include laser ablation and desorption of graphite or fullerene targets [MS Dresselhaus et al., “Science of Fullerenes and Carbon Nanotubes”, Academic Press, San Diego, Calif., 1996; HD Busmann et al., “Surface Science”, 272: 146, 1992], fission fragments impact on a C60 coated surfaces [K Baudin et al., “A Spontaneous Desorption Source For Polyatomic Ion Production”, Rapid Comm. in Mass Spect. 12 (13): 852-856, 1998], fullerene thermal desorption combined with electron attachment or electron impact ionization [T Jaffke et al., “Formation of C60− and C70− By Free Electron Capture. Activation Energy And Effect of the Internal Energy On Lifetime”, Chem. Phys. Lett. 226: 213-218, 1994; SCC Wong et al. “Development Of A C-60(+) Ion Gun for Static SIMS and Chemical Imaging”, Appl. Surf. Sci. 203: 219-222, 2003; D Weibel et al., “A C-60 Primary Ion Beam System For Time of Flight Secondary Ion Mass Spectrometry: Its Development and Secondary Ion Yield Characteristics”, Anal. Chem. 75 (7): 1754-1764, 2003]. Attempts have also been made to use conventional ion sources (arc-discharge and sputtering type) [PD Horak et al., “Broad Fullerene-Ion Beam Generation and Bombardment Effects”, Applied Physics Letters, 65 (8): 968-970, 1994; S Biri et al., “Production of Multiply Charged Fullerene and Carbon Cluster Beams by a 14.5 GHz ECR Ion Source”, Review of Sci. Instr. 73(2): 881-883, 2002; C Sun et al., “Extraction of C60− and Carbon Cluster Ion Beams from a Cs Sputtering Negative Ion Source”, Fudan Univ., Shanghai, Peop. Rep. China. Hejishu 17(7): 407-410, 1994].
These methods have various drawbacks when used for submicron focused beam applications. Among these are the complexity of the source, the need for an additional mass filter due to fragmentation upon ionization, low current density and brightness, and large energy dispersion of emitted ions or poor focusing.
There is thus a widely recognized need for, and it would be highly advantageous to have an apparatus and method for the generation of a positively charged ion beam by field ionization, devoid the above limitations.